The present invention relates to a light amount adjustment method for use in an image exposure apparatus, in particular, a light amount adjustment method for use in an image exposure apparatus with which image quality is enhanced by suppressing degradation of image quality in the rising and falling portions of a light beam through light amount increase in the edge portions of an image signal.
In recent years, a computer-to-plate (CTP) system has been used widely in which a digital image is created using a computer and image recording is performed directly on a printing plate without using a film at the time of plate making. As an image exposure apparatus used in such a CTP system or the like, an image exposure apparatus is used which records an image as a latent image by performing laser light exposure in accordance with a digital image signal while holding a photosensitive material, such as a PS plate, on the surface of a rotating drum.
With this technique, the turned-on state of a light beam irradiated from a light source for exposure is controlled in accordance with a binary image signal generated based on the image data of an original image to be recorded and a photosensitive material is two-dimensionally scanned with the light beam modulated in accordance with the image signal by relatively moving the light source for exposure and the photosensitive material. In this manner, a desired image is recorded as a latent image on the photosensitive material.
An image for plate making is a halftone dot gradation image (area-modulated image) using so-called halftone dots and each halftone dot is recorded by a collection of many dots formed through scan-exposure using a light beam having a certain size determined in accordance with a resolution.
Conventionally, in such an exposure apparatus, when the quality of a laser beam is degraded, a correction has been made in which image quality is improved by creating an edge signal from the ON/OFF signal for a laser beam and changing a light amount at the instant of laser beam ON/OFF in accordance with the edge signal. The increase of the light amount at the instant of laser beam ON/OFF in accordance with the edge signal created from a laser beam-ON/OFF signal will be referred to as the “edge light amount increase” in this specification.
For instance, a method is proposed with which the brightness of a light source is increased at the time of exposure of a contour portion of an image to be recorded on a photosensitive material and the edge of density is sharpened in the contour portion through edge enhancement based on differentiation of a binary signal (see JP 08-023422 A, for instance).
In the conventional edge light amount increase process, the light amount increment has been determined to a uniformly fixed value with reference to a result of evaluation conducted in advance and the edge light amount increase has been carried out on a fixed percent basis using the value. Therefore, when the sensitivities or the like of photosensitive materials vary from one lot to another or from one kind to another, there occurs a problem in that the light amount correction by the edge light amount increase becomes insufficient or, inversely, excessive.